1. Field of the Invention
This invention relates to the improvement of a solvent used in non-aqueous solvent system lithium ion secondary cells.
2. Description of the Related Art
Attempts have been made to improve the temperature properties of the electrolyte in non-aqueous solvent system lithium ion secondary cells, and to raise its boiling point to render it non-flammable. For example, in Japanese Patent Laid-Open Publication No. Hei 5-326017, a high boiling non-aqueous solvent such as sulfolane is added to an electrolyte to raise the boiling point so as to make the electrolyte non-flammable.
However in that art, the high boiling non-aqueous solvent also has a high viscosity. When this is added to the electrolyte, the electrolyte composition is largely modified, its viscosity increases, and tends to lead to a decrease of cycle properties due to a decrease of electrical conductance or a decrease of ion transport rate.